1. Field of the Invention
The present invention relates to a method and apparatus for demoisturizing air, more particularly to a method and apparatus for adsorbing humidity from and heating drying air circulated for drying a synthetic plastic material to be molded by a molding machine. The apparatus is adapted principally to be used in association with a molding machine, such as an injection molding machine, a blow molding machine and an extrusion molding machine.
2. Description of the Related Art
Many synthetic plastic materials absorb moisture and thus will normally contain anywhere from a fraction of a percent to a few percent of water. When a synthetic plastic material is molded by a molding machine, it is usually necessary to remove the water component from the plastic material prior to supply into the molding machine.
Accordingly, a drying hopper is conventionally used to first dry the synthetic plastic material. Hot air or hot demoisturized air is passed from a lower end of the drying hopper containing the moist plastic material, adjacent to a hopper outlet for delivering the dried plastic material, to an upper end adjacent to a hopper inlet for receiving supplementary moist plastic material.
Whether hot air or hot demoisturized air is used as the drying air is determined based on the properties of the plastic material, i.e., equilibrium water content rate under a predetermined temperature. There are several kinds of plastic material, such as polyamide and polyethylene terephthalate, which need to be dried by hot demoisturized air. The supply of the hot demoisturized air to the drying hopper is conventionally conducted by an air-adsorption treatment apparatus which includes an adsorption treatment means retaining therein an adsorbent or combination of adsorbents. The air-adsorption treatment apparatus receives the drying air returned from the upper end of the drying hopper to adsorb moisture from the returned drying air an by adsorption treatment chamber. The apparatus heats up the drying air to a required hot temperature and then resupplies the hot demoisturized drying air into the lower end of the drying hopper. That is, the drying air is repeatedly circulated through the drying hopper and the air-adsorption treatment apparatus.
At this stage, since the drying air retains a higher than normal temperature even when it is discharged from the upper end of the drying hopper after drying of the synthetic plastic material, it is conventionally preferred to employ synthetic zeolite as an adsorbent, which has an excellent adsorbing property at a relatively high temperature range. The adsorbing property of the adsorbent, however, gradually deteriorates with the progress of the adsorbing treatment. This is because the adsorbent is gradually humidified with moisture.
Therefore, the air-adsorption treatment apparatus must regenerate the adsorbent humidified with moisture during the treatment of the drying air. Regeneration of the moisture-saturated adsorbent is carried out by desorbing moisture from the adsorbent by the employment of hot air introduced from the atmosphere through a blower and a heater and then by cooling the adsorbent by the employment of cool air, also introduced from the atmosphere, until the adsorbent becomes cold enough for recovering the adsorbability thereof.
One example of the above-mentioned conventional air-adsorption treatment apparatus is disclosed in U.S. Pat. No. 3,972,129 to Graff. However, the apparatus of Graff has to take into account that, while the regeneration of the adsorbent is carried out, the drying air must be prevented from flowing through the drying hopper containing therein synthetic plastic material to be delivered to an injection molding machine. That is, the air-adsorption treatment cycle is interrupted.
Moreover, the apparatus of this example requires a special blower which can be operated in opposite directions in order to realize alternative air-adsorption treatment and regeneration of the adsorbent in a single circuit.
There are, however, some cases where interruption of the air-adsorption treatment cycle is unfavorable. Further, in the regeneration cycle, the adsorbent dried by the hot air is exposed to cooling air introduced from the atmosphere. As a result, the dried adsorbent might be moistened by humidity contained in the cooling air and, accordingly loses a part of its adsorbability.
Another example of the conventional air-adsorption treatment apparatus is illustrated in FIG. 1, in which two adsorbent towers are alternatively operated so as to continually demoisturize and heat drying air for drying synthetic plastic material in a drying hopper without any interruption.
Referring to FIG. 1, a drying hopper section enclosed by the broken line designated by reference numeral 1 includes a drying hopper 2 which can store synthetic plastic granulate or pellets 3 to be supplied to a molding machine (not illustrated in FIG. 1). The air-adsorption treatment apparatus is arranged in association with the drying hopper section 1 and includes two adsorbing towers 7 and 8 containing therein adsorbents 9 and 10, respectively, and also accommodating respective heaters 11 and 12 for regenerating or desorbing hot air of, for example, 250.degree. C. through 300.degree. C.
The apparatus also includes air circuits for alternatively connecting the two adsorbing towers 7 and 8 to the drying hopper 2, a filter 4, a blower 5 for propelling drying air having returned from the drying hopper 2 via the filter 4 toward one of the adsorbing towers 7 and 8, a blower 6 for introducing air from the atmosphere for the desorbing purpose, a heater 13 for heating up the drying air to a required temperature before entering the drying hopper, and two automatically operated valves 14 and 15 in the form of four-way change-over valves. The changed-over states of the two valves 14 and 15 are also illustrated and designated by reference numerals 14' and 15', respectively. Arrows 16 in FIG. 1 indicate the flow of the drying air for drying the plastic pellet 3. The drying air returning from an upper end of the drying hopper 2 flows through the cleaning filter 4 into the blower 5 where it is propelled so as to flow toward the automatically operated valve 14. Subsequently, the drying air passing through the valve 14 flows into the adsorbing tower 7, via the heater 11, which is not energized during the adsorbing cycle. Within the adsorbing tower 7, the drying air humidified with moisture removed from the plastic pellets 3 is subjected to adsorbing treatment by the adsorbent 9. Thereafter, the drying air flows, via the valve 15, into the heater 13 where it is heated up to a temperature ready for drying the plastic pellets 3 and then enters the drying hopper 2.
In the same period of time as the abovementioned adsorbing cycle, the other adsorbing tower 8 is in a regeneration cycle. Arrows 17 indicate the flow of air introduced from the atmosphere by the blower 6. The introduced air flows via the automatically operated valve 14 into the heater 12, which is in energization, so that it is heated and becomes hot air. The hot air subsequently passes through the adsorbent 10 so as to desorb moisture from the adsorbent 10. The air then flows out of the circuit into the atmosphere via the automatically operated valve 15. After a predetermined period of time, the heater 12 is deenergized. Therefore, air introduced by the blower 6 is not heated and acts so as to cool the adsorbent 10 in the adsorbing tower 8 until the adsorbent 10 is regenerated. When the desorbing and cooling operations to regenerate the adsorbent 10 are completed, the two automatically operated valves 14 and 15 are changed over to the positions 14' and 15', respectively. Accordingly, the adsorbing tower 7 is now subjected to the regenerating cycle, while the adsorbing tower 8 undertakes the adsorbing cycle for demoisturizing the drying air.
With the air-adsorption treatment apparatus of FIG. 1, it should be noted that each of the adsorbing towers 7 and 8 is passed by the regenerating air introduced from the atmosphere in the same direction as the drying air. It is, however, necessary for the drying air to be demoisturized by the adsorbing tower 7 or 8 to the extent that the dew point of the drying air is low enough (for example, -40.degree. through -60.degree. C.) for drying synthetic resin material when the air leaves the adsorbing tower 7 or 8. Accordingly, the outlet side of each of the adsorbing towers 7 and 8 must be more adsorbable than the inlet side of each of the towers 7 and 8. Taking this fact into consideration, the regenerating hot air should pass through the adsorbing tower 7 and 8 in the direction reverse to that in which the drying air flows.
Further, in the apparatus of FIG. 1, cooling of the adsorbing towers 7 and 8 must be carried out by air that is introduced from the atmosphere by the blower 6. The cooling air unavoidably contains humidity that may adversely affect the adsorbing towers 7 and 8. In addition, during the cooling of the adsorbing towers 7 and 8, the humid cooling air per se is subjected to the adsorbing treatment, which generates latent heat in the towers 7 and 8. Such latent heat has an additional adverse effect on the cooling of the adsorbing towers 7 and 8. Moreover, the cooling air removes heat from the adsorbing towers 7 and 8 and dissipates the heat into the atmosphere. That is, waste of heat energy occurs. Consequently, the air-adsorption treatment apparatus of FIG. 1 is imperfect from the viewpoint of industrial efficiency.